Heat-resistant steels used as high temperature and high pressure-resistant materials in thermal power plants, chemical plants, nuclear power plants, etc., can be broadly classified into austenitic stainless steels and ferritic heat-resistant steels such as a Cr--Mo steel, a Mo steel and a carbon steel. Suitable materials are selected from these heat resistant steels from the aspects of the temperature of the high temperature and high pressure portions, environments and economy.
Among the heat-resistant steels described above, the austenitic stainless steels are most excellent in high temperature strength and the corrosion resistance but it have a large coefficient of linear expansion and a small heat transfer rate. Also, they are susceptible to stress corrosion cracking. Further, they are expensive because the amounts of addition of alloy elements such as Cr, Ni, etc., are large. Therefore, Cr--Mo steels as ferritic heat-resistant steels have been employed in most cases as the high temperature and pressure-resistant members described above with the exception of the case where the temperature of use is not lower than 600.degree. C. or the environment of use is a remarkably corrosive environment. Among the Cr--Mo steels, a Cr--Mo steel having a Cr content of about 1% has high economy, though its high temperature resistance and corrosion resistance are inferior, in comparison with a Cr--Mo steel having a Cr content of at least 2%. On the other hand, it has a higher elevated temperature strength and higher oxidation resistance than the Mo steel and the carbon steel, thought its cost is higher.
A typical example of the material of the Cr--Mo steel having the Cr content of 1% and having such features includes STBA23 (1.25 Cr--0.5 Mo) and STBA22 (1 Cr--0.5 M) according to the JIS standards. These steels can be used at temperatures of up to about 550.degree. C. from the aspect of the oxidation resistance due to their Cr contents. However, since their creep rupture strength is lower than that of the Cr--Mo steel having a Cr content of at least 2%, the thickness must be large and thus the economy is inferior to the Cr--Mo steel having a Cr content of at least 2%.
Therefore, their application range is limited to a pressure-resistant member within a temperature range of 400 to 500.degree. C. In other words, the temperature range of use of the Cr--Mo steel having a Cr content of 1% can be drastically expanded if the high temperature strength of this steel can be improved. From this aspect, the improvement in the strength of the Cr--Mo steel having a Cr content of 1% as a high temperature and high pressure resistant material of the thermal power plants, etc., is necessary.
Though the industrial effect brought forth by the improvement in the strength of the Cr--Mo steel having a Cr content of about 1% is great as described above, the prior art technologies involve the problem that the improvement in the strength invites deterioration of toughness and machinability. The Cr--Mo steel such as STBA23 of the JIS Standards, for example, improves the high temperature strength by solid solution strengthening of Mo and precipitation strengthening of fine carbides such as Cr, Fe and Mo. When these additional elements are alone used, however, pro-eutectoid ferrite exceeds 50%, a sufficient tensile strength cannot be obtained in an intermediate temperature range, coarsening of carbides is quick, and a long-term creep strength cannot be sufficiently obtained.
On the other hand, Japanese Examined Patent Publication (Kokoku) No. 63-18038 discloses a low alloy steel having excellent creep characteristics and excellent hydrogen permeation resistance. However, though at least 0.75% of Mo and at least 0.65% of W are substantially added in addition to the Cr content of at least 2%, this prior art does not at all consider weldability of the steel which is very important for utilization and machining. Further, the material of this reference is subjected to annealing treatment at a temperature of 1,050.degree. C. to increase the strength, but in the case of heat transfer pipes of the thermal power plant, there occur many cases where water cooling annealing cannot be carried out from the aspect of heat-treatment. Therefore, the steel yet has a problem in working.